Code Division Multiple Access (CDMA) separates signals from different users with unique code sequences, so that the users can transmit simultaneously using the same frequency. On receipt of a signal, a wireless receiver multiplies the signal with a dispreading waveform to recover a transmitted data sequence. However, data recovery is only possible if the code sequence of the received signal is synchronised with the dispreading waveform.
Wideband Code Division Multiple Access (WCDMA) uses orthogonal spreading codes followed by non-orthogonal pseudo-random scrambling codes. Orthogonal Variable Spreading Factor (OVSF) codes are used in Universal Mobile Telecommunication System (UMTS) and High-Speed Downlink Packet Access (HSDPA) protocols. In the HSDPA mode of UMTS, a user is allocated more than one OVSF code.
In a single path propagation channel, simple dispreading can recover symbols transmitted by a user, since other OVSF codes are orthogonal thereto. However, in multi-path propagation channels, a signal received by a wireless receiver is a superposition of scaled and shifted copies of the originally transmitted signal. Thus, it is difficult to prevent the spreading codes assigned to different users from interfering with each other. More particularly, if a received signal only comprised the codes of a given user, the wireless receiver could use joint detection algorithms to estimate the transmitted symbols. However, when a signal comprises codes from multiple users, a wireless receiver must decipher a communication from a user, being aware only of the codes uniquely allocated to that user (i.e. unaware of the other codes).
International Patent Application WO2005/109708 describes a method for determining spreading codes. However, the spreading codes detected in WO2005/109708 are not the actual spreading codes. Instead, they are virtual codes and the interference reconstructed there from is not an exact interference but an approximation of the projection of the interference on the virtual codes. U.S. Pat. No. 6,956,893 describes a parallel interference cancellation technique. However, the technique does not provide for OVSF symbol detection, since it uses single length spreading parent codes instead of actual child codes. Furthermore, the technique does not use joint detection, since it is designed for single code CDMA.